Properties of sulfonated poly(butylene terephthalate)

The synthesis, morphology, and mechanical properties of sulfonated poly(butylene terephthalate) (PBT) and its unsulfonated analogs were studied. The morphology of these copolymers crystallized from the melt were examined by a combination of wide-angle x-ray scattering (WAXS), polarized light microscopy, and small-angle light scattering (SALS). Stress-strain measurements are correlated with the morphological results. Spherulitic morphology, with a maltese cross at 45°C with respect to the crossed polars, is formed at low sulfonate levels (≤ 5.0 mol %). At a higher ion content, the maltese cross rotates 45° to form a cross pattern. At still higher sulfonate contents, typically 13 mol %, the light scattering pattern disappears completely. Microscopic and WAXS examination of these functionalized PBT copolymers confirms that the crystallinity level decreases with increasing ion content and is eliminated completely at the higher sulfonation level. The spherulite radius, however, remains invariant until the highest functionalization level. On the contrary, the morphology and properties of the unsulfonated isophthalate copolymer analogs remain relatively constant over the entire composition range examined. In several compositions clearly inferior properties are noted compared with the ion-containing copolymers.     

Analysis of Polymorphism and Dual Crystalline Morphologies in Poly(hexamethylene terephthalate)

Summary: The polymorphisms in poly(hexamethylene terephthalate) (PHT), along with their associated melting and spherulite morphologies, were examined by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), and polarized‐light microscopy (PLM). The morphology and crystal cells were dependent on the temperature of crystallization. When melt‐crystallized at low temperatures (90–135 °C), PHT showed at least five melting peaks and two re‐crystallization peaks upon DSC scanning, and the samples displayed various fractions of both α and β crystals. However, only a single melting peak was obtained in PHT melt‐crystallized at 140 °C or above, which displayed a single type of β crystal. In addition, two different forms of spherulites were identified in melt‐crystallized PHT, with one being a typical Maltese‐cross spherulite containing the α crystal, and the other being a dendrite‐type packed mainly with the β crystal. This study provides timely evidence for a critical interpretation of the relationship between multiple melting and polymorphisms (unit cells and spherulites) in polymers, including semi‐crystalline polyesters.

WAXD diffractograms for PHT melt‐crystallized at 140 °C, revealing a single type of β‐crystal cell.     

La cristallisation du polydioxolanne

Crystallization kinetics have been studied for two samples of polydioxolan (molecular weights 10.000 and 30.000). The crystallization was followed at temperatures between 0 and 21°C in a DSC calorimeter. The results obey Avrami's equation. The Avrami exponent was found to be two indicating a spontaneous and probably heterogeneous mode of nucleation. Over the temperature range studied, the crystallization lead to the growth of two-dimensional spherulites.The morphology of polydioxolan samples was studied by optical microscopy and small-angle light scattering. Samples crystallized in liquid nitrogen are made of small spherulites of the order of 5 μm. Samples crystallized between 0 and 21°C are made of large spherulites, of the order of 1 mm. Samples crystallized at 25 and 35°C show large and “abnormal” spherulites, made of two optical phases corresponding to the centre and the perimeter.No difference was seen between the morphologies of the two samples studied. In both cases, the time of half-crystallization was the same when plotted as a function of the degree of supercooling. Equilibrium melting points of 79 and 85°C were found for the low and high molecular weight samples, respectively.     

Crystallization of isotactic polypropylene and high-density polyethylene under negative pressure resulting from uncompensated volume change

The melting behavior of spherulites in thin sections of isotactic polypropylene bulk samples and high-density polyethylene thin films crystallized isothermally at various temperatures has been studied by polarized light microscopy. The regions around cavities and multiple boundary points between spherulites have higher melting temperatures than the other parts of spherulites crystallized in Regime III. The increase in melting temperature is explained as a result of crystallization under negative pressure arising locally in pockets of occluded melt due to density change during spherulitic crystallization. The negative pressure lowers locally the equilibrium melting temperature and therefore decreases the undercooling, which results in an increase in lamellar thickness. Sectioning of bulk samples releases frozen negative pressure and reveals the increase in melting temperature of those parts of spherulites that were crystallized at lower undercooling. © 1993 John Wiley & Sons, Inc.     

Ring‐banded spherulites in poly(pentamethylene terephthalate): A model of waving and spiraling lamellae

A new aryl polyester, poly(pentamethylene terephthalate) (PPT) with five methylene groups in the repeat unit, was synthesized. Its multiple‐melting behavior and crystal structure were analyzed with differential scanning calorimetry and wide‐angle X‐ray diffraction. In addition, the spherulitic/lamellar morphology of melt‐crystallized PPT was investigated. Typical Maltese‐cross spherulites (with no rings) were seen in melt‐crystallized PPT at low temperatures (70–90 °C), but ring patterns were seen in PPT crystallized only at temperatures ranging from 100 to 115 °C, whereas rings disappeared with crystallization above 120 °C. The mechanisms of the rings in PPT were explained with several coordinated directional changes (wavy changes, twisting changes, and combinations) in the lamellae during growth. Scanning electron microscopy, in combination with atomic force microscopy, further proved that the ringed spherulites originated from the aggregation of sufficient numbers of edge‐on lamellar crystals; the radial‐growth edge‐on/flat‐on lamellae could be twisted and/or waved to form realistic band patterns. A postulated model properly described a possible origin of the ring bands through combined mechanisms of waving (zigzagging) and twisting (spiraling) of the lamellae during crystallization. Superimposed twisting and/or wavy models during crystallization were examined as examples. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4421–4432, 2004     

THE MORPHOLOGY AND STRUCTURE OF POLYETHYLENE TEREPHTHALATE SPHERULITES

The morphology and structure of three types of normal and abnormal spherulites of polyethylene terephthalate cast from solution in dimethylphthalate were studied by polarizing microscopy, transmission electron microscopy and electron diffraction techniques. In the normal negative spherulites the [0]~* direction is parallel to the radius direction. In the normal positive spherulites the radius direction is parallel to the [6]~*. In the abnormal PET spherulites the Maltese cross extinction pattern in the polarizing microscope under crossed polars is oriented at 45°to the polars and there are concentric extinction rings around the center of the spherulite. Electron diffraction pattern indicates that [2]~* is parallel to the radius of the spherulite and this explains the observed extinction pattern, in the abnormal spherulite.     

Fractional crystallization behavior of PCL and PEG in blends

The crystallization behavior of poly(e-caprolactone)/poly(ethylene glycol) (PCL/PEG) blend was investigated by differential scanning calorimetry (DSC) and polarized microscopy (POM). Individual phase transition peaks in the DSC curves for both PEG and PCL in all the polymer blends with different PCL contents were observed. The crystallization and melting peak temperatures of PEG were at 41 and 65°C, respectively; while the crystallization and melting temperatures of PCL located at 28 and 56°C, respectively. In-situ POM results demonstrated that spherulites crystalline morphology was formed for both PCL and PEG homopolymers. In PEG/PCL blend, however, both the phase separation morphology and spherulitic morphology can be observed. In blends with 30 or 50 wt % PCL, the PCL component formed dispersed phase and crystallized at lower temperature. However, in blends with 70% PCL, the phase inversion behavior occurred. The continuous PCL phase crystallized at 35°C, while the PEG dispersed phase crystallized at a lower temperature. Fractional crystallization behavior of PEG and PCL was controlled by temperature. The spherulites growth rate of PEG was greatly influenced by temperature, instead of the content of PCL component in the PCL/PEG blends.     

Preparation of small poly(butylene terephthalate) spheres by crystallization from solution

The crystallization of poly(butylene terephthalate) (PBT) from moderately dilute solutions of PBT in a diglycidyl ether of bisphenol-A epoxy has been investigated. PBT dissolves in this epoxy approximately 35°C below its usual melting temperature of 227°C to form a one-phase solution. Cooling this solution below 165°C leads to rapid crystallization of the PBT. The resulting mixture of liquid epoxy and crystalline PBT has a low viscosity and contains highly birefringent, individual PBT spherulites. The PBT spherulites have a narrow size distribution and a high surface-to-volume ratio. These particles are suggested to arise from a rapid crystallization that follows liquid–liquid phase separation. © 1994 John Wiley & Sons, Inc.     

MULTIPLE MELTING AND CRYSTALLIZATION BEHAVIOR OF NYLON 1212

The wide-angle X-ray diffraction (WAXD) patterns of isothermally crystallized Nylon 1212 show that γ-form crystals form below 90℃ and the α-form crystals can exist above 140℃. In the temperature range of 90-140℃, the α-form and γ-form crystals coexist. Variable-temperature WAXD exhibits that the nylon 1212 γ-form does not show crystal transition on heating, while α-form isothermally crystallized at 160℃ exhibits Brill transition at a little higher than 180℃ on heating. The multiple melting behaviors of Nylon 1212 isothermally crystallized from melt come from a complex mechanism of different crystal structures, dual lamellar population and melting-recrystallization. In polarized optical microscope (POM) observations, Nylon 1212 isothermally crystallized at 175℃ shows the ringed banded spherulites. However, at temperatures below 160℃ the ringed banded image disappears, and cross-extinct spherulites are formed.     

Effect of birefringence on light scattering from deformed spherulitic polymer films

Light scattering from oriented samples of crystalline polymers is affected by the birefringence of the sample. An extension of the theory for scattering from uniaxially deformed two-dimensional and three-dimensional spherulites is made so as to include the retardation of the incident and scattered beams in passing through the birefringent sample. Strain influences scattering, in that it changes the birefringence of the sample and it also changes the anisotropy and shape of the spherulites. Scattering intensities are calculated for both crossed and parallel polarizers as a function of Ω, χ, and Φ, where Ω is the angle between the stretching direction of the sample and the horizontal direction, and χ and Φ are the angles between the stretching direction and the polarization directions of the polarizer and analyzer, respectively. It is shown that for crossed polarizers with Φ = 45° and χ = 45° birefringence changes largely influence the results but that for the polarizers parallel at Φ = 0° and χ = 0° or crossed at Φ = 90° and χ = 0° the birefringence effect is minimized. The intensity distributions for crossed polarizers at Φ = 45° and χ = 45° from polyethylene films stretched to give retardations up to several wavelengths, are found to be in good agreement with the calculated results.     

Crystalline morphology of a thermotropic aromatic polyester crystallized from nematic melt

The crystalline morphology of a thermotropic aromatic polyester crystallized from a nematic melt was investigtated by means of polarized optical microscopy (POM) and scanning electron microscopy (SEM). Due to POM measurements it was found that spherulites of two different types are formed within the two different temperature regions. When T_c was exceeding 170°C, spherulites of type‐1 showing a negative birefringence grew with a radial fibrillar morphology and exhibited a clear Maltese‐cross pattern. The radius growth rate of type‐1 spherulites was about 2.2 μm/min at 185°C. When T_c was smaller than 160°C, spherulites of type‐2 were formed and exhibited a radially outward growing structure but no evident Maltese‐cross pattern. These spherulites could be seen by the naked eyes due to their size reaching several millimeters. SEM observations revealed that the spherulites of type‐1 exhibited a ripple‐like homocentric morphology with periodical compact fibrils having a diameter of about 150 nm perpendicular to the radial direction. In contrast, the spherulites of type‐2 exhibited, as apparent from performed SEM images, radially growing crystallites of about 500 nm in size with no periodicity in the radial direction.     

Light scattering studies of mixtures and fractions of linear polyethylene

Previous work on the small-angle light scattering of polyethylene films, to determine the supermolecular structure, has been continued. One of the main efforts has been the study of a binary mixture whose low molecular weight component forms well defined spherulites and whose high molecular weight component yields a poorly defined rod-like morphology. The addition of the high molecular weight fraction causes a progressive deterioration of the initial spherulitic morphology; a relatively small amount of the high molecular weight species causes a major decrease in the spherulitic size. However, there are no indications of any spherulitic structures when the weight fraction of the high molecular weight species is 0.5 or greater. The isothermal crystallization of a fraction M = 6.6 × 10⁵ was also studied. Spherulites were formed at low crystallization temperatures while at the higher crystallization temperatures the morphology became nondistinct. Preliminary studies with solvents indicate that high molecular fractions, which do not form spherulites when crystallized in the pure state, do so when crystallized from highly swollen solutions.     

Periodic extinction bands composed of all flat‐on lamellae in poly(dodecamethylene terephthalate) thin films crystallized at high temperatures

Using in‐house synthesized poly(dodecamethylene terephthalate) (P12T) as a model, periodic extinction‐banded spherulites melt‐crystallized at high T_cs (100–115 °C) are expounded in terms of growth mechanism. The extinction‐banded spherulites wildly differing from the usual blue/orange double ring‐banded spherulites are composed of all flat‐on discrete single‐crystalline lamellae packed like roof shingles (or fish scales) along the circularly curved bands and the lamellae in the extinction bands are flat with a lozenge shape with no continuous twisting at all. For P12T films of more than 10 µm crystallized at T_c = 105–115 °C, no periodic bands were seen, and all spherulites were ringless, where periodic growth precipitation of crystals to extinction does not occur until impingement. Extinction bands in the P12T spherulites with the inter‐ring spacing steadily decrease with decreasing film thickness, because for thinner films (submicrons to 2 µm), draining or depletion of available molten species takes place more frequently, leading to bands of smaller inter‐ring spacing. The petal‐like extinction bands are discussed and analyzed in detail using 3D AFM imaging. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 601–611     

Melting behavior,crystallization kinetics,and crystal structure of poly(2‐hydroxyethoxybenzoate)

The melting behavior and crystallization kinetics of poly(2‐hydroxyethoxybenzoate) (PHEBA) were investigated with differential scanning calorimetry and hot‐stage optical microscopy. The observed multiple endotherms, commonly displayed by polyesters, were influenced by the crystallization temperature. By the application of the Hoffman–Weeks method to the melting temperatures of isothermally crystallized samples, a value of 232 °C was obtained for the equilibrium melting temperature. Isothermal crystallization kinetics were analyzed according to Avrami's treatment. Values of Avrami's exponent n close to 3 were obtained, independently of the crystallization temperature, in agreement with a crystallization process originating from predetermined nuclei and characterized by three‐dimensional spherulitic growth. In fact, space‐filling banded spherulites were observed by hot‐stage optical microscopy at all crystallization temperatures explored, with the band spacing increasing with increasing crystallization temperature. The rate of crystallization became lower as the crystallization temperature increased as usual at low undercooling, with the crystallization process controlled by nucleation. The equilibrium heat of fusion was determined by differential scanning calorimetry and wide‐angle X‐ray scattering measurements. Finally, the crystal phase of PHEBA was investigated with wide‐angle X‐ray scattering, and a triclinic unit cell was hypothesized. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1354–1362, 2002     

A New Crystal Form,Polymorphism, and Multi‐Morphology in Biodegradable Poly(3‐hydroxypropionate)

Summary: A new crystal morphology (δ form) of poly(3‐hydroxypropionate) (PHP) is found in cast and melt‐crystallized PHPs with low molecular weight, in which the PHP chains possibly adopt a 2₁ helix rather than the trans conformation found in the β or γ form. The fusion temperature‐ and the crystallization temperature‐dependent polymorphism are responsible for the dual morphologies and the unique growth kinetics of spherulites in the melt‐crystallized PHPs.

a) A dual‐morphology developed at 70 °C in PHP films after melting at 117 °C and b) that formed during cooling at a rate of 1 °C · min⁻¹ from 130 °C.     

Studies on the biaxial stretching of polypropylene film. III. Electron microscopic observation of the one-step biaxial stretching of isotactic polypropylene spherulites

Polypropylene films of various isotacticities and crystallinities were stretched biaxially in one step in air at 140–152°C or polyaxially in poly(ethylene glycol) at 130–160°C, and the morphological changes were studied by electron microscopy (replica). In the initial stage of stretching, with v_A = 1.4, the spherulites of one of the films used for the experiment were broken both from the centers and boundaries, and those of another film were broken mainly from the center. This difference in the deformation behavior seems to be characteristic of the film properties and independent of the method of stretching, although the factors involved are still unknown. On further stretching (v_A = 22), well annealed spherulites were broken into many small blocklike fragments with unfolded fibrils running among them, particularly at the low stretching temperature (140°C), and fibrillation proceeded at the expense of the residual fragments. In the case of quenched or slightly crystallized material, the fragments were dendritic and divided into finer and finer fibrils on stretching. At elevated temperature, however, even for well annealed spherulites, the deformation behavior resembles that of the quenched material, and at a high degree of stretching the spherulites take on the fibrillar net structure in every case. In films containing a high amount of atactic fraction, radial, tangential, and boundary cracking occurred more easily, and broad fibrils were observed across the cracks.     

Investigations of the equilibrium melting temperature in PBT and PC/PBT blends

The melting behavior of poly(butylene terephthalate) and its blends with bisphenol-A polycarbonate was investigated with differential scanning calorimetry. The aim of this work was to determine the equilibrium melting temperature and its dependence on the blend composition using the Hoffman-Weeks plots. It is shown that the critical analysis of various influences on the melting peak is necessary for the reorganization processes and crystallized content of blends. The experimental conditions and the corrections of measured temperatures were derived and discussed. It was found that the use of the extrapolated onset temperature T_m,o of the melting peak is more efficient than the maximum temperature T_m for the Hoffman-Weeks plots. The equilibrium values of pure PBT are determined to be T^o_m,o = 501 K and T^o_m = 506 K. The equilibrium temperatures of the blends do not show a depression with increasing PC content. Using the Nishi-Wang relation, the results can be qualitatively interpreted with a polymer-polymer interaction coefficient χ ≥ 0 between both components. A weak increase in the equilibrium temperature with increasing PC content was observed. A hypothesis to explain this is based on the possibility of a changed population of the different spherulites with various melting temperatures in dependence on PC content. © 1996 John Wiley & Sons, Inc.     

Thermal properties and influence of orientation on crystalline morphologies in stereoblock polypropylene and related elastomers

Atomic force microscopy (AFM), small angle X‐ray scattering (SAXS), temperature modulated differential scanning calorimetry (TMDSC), variable heating rate DSC, an independent rapid heating rate method for melting points, and cyclic mechanical testing were used to study semicrystalline thermoplastic elastomeric polypropylenes (ELPPs) and related semicrystalline polyolefins including ethylene copolymers. Low crystallinity (ca., 9 and 15%) ELPP samples were studied by AFM in the nonoriented and melt‐oriented states. AFM images taken as a function of time after quenching of a melt‐drawn and highly nucleated film resolved details of secondary crystallization involving lateral growth on the ordered row‐nucleated structures. For nonoriented films, isothermal melt crystallization at high temperatures (110 °C) led to similar features for the two ELPPs. The dominant crystalline morphology studied by AFM consisted of small (several nm in width) granular crystallites organized into immature but large spherulites spanning tens of microns. A striking cross‐hatch morphology was detected in regions of the surface in 110 °C crystallized samples, which is contrasted with melt‐drawn films where row nucleated structures dominated the morphology in the film under no external stress. AFM was also used to monitor the morphological changes that occurred as the films were stretched at 25 °C. Break‐down of lamellae was observed, resulting in oriented narrow fibrils. Cyclic stress‐strain curves showed the expected result where lower crystallinity ELPPs had higher recoverable levels of set after both 100 and 500% elongation. TMDSC was used to resolve the broad melting and recrystallization regions in these low to medium crystallinity ELPP systems, and to contrast the results with ethylene copolymers. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

New understanding on the memory effect of crystallized iPP

In this study, recovery processes of isotactic polypropylene(iPP) melted spherulites at 135 °C after melting at higher temperatures(170 °C–176 °C) were investigated with polarized optical microscopy and Fourier transform infrared spectroscopy. The recovery temperature was fixed to exclude the interference from heterogeneous nuclei. After melting at temperatures between 170 °C and 174 °C, the melted spherulite could recover back to the origin spherulite at low temperatures. Interestingly, a distinct infrared spectrum from iPP melt and crystal was observed in the early stage of recovery process after melting at low temperatures, where only IR bands resulting from short helices with 12 monomers or less can be seen, which indicates that the presence of crystal residues is not the necessary condition for the polymer memory effect. Avrami analysis further indicated that crystallization mainly took place in melted lamellae. After melting at higher temperatures, melted spherulite cannot recover. Based on above findings, it is proposed that the memory effect can be mainly ascribed to melted lamellae, during which crystalline order is lost but conformational order still exists. These conformational ordered segments formed aggregates, which can play as nucleation precursors at low temperatures.